Login Sequence Diagram

 Diagramatic representation of the  interaction between processes in their own order is
a Sequence Diagram.
A simple Sequence diagram for login is shown below:

Short descriptions on notations used in the above diagram.

Actor:
  An actor is an entity that interacts with the system
(can be human or any other process or entity that interacts with the system  ).

Call Message :
 Represents the forward communication between the entities .

Return Message :
Represents the message passed back to the caller.